A chuck is a specialized type of clamp used to hold rotating tools or materials.
A self-centering chuck uses dogs (usually called jaws), interconnected via a scroll gear (scroll plate), to hold onto a tool or workpiece. Because they most often have three jaws, the term three-jaw chuck without other qualification is understood by machinists to mean a self-centering three-jaw chuck. The term universal chuck also refers to this type. These chucks are best suited to grip circular or hexagonal cross-sections when very fast, reasonably accurate (±0.005 in TIR) centering is desired.
Sometimes this type of chuck has four or six jaws instead of three. More jaws confer more secure grip (if the work is truly cylindrical) and thin-walled work will deform less. Four jaws are also useful for square bar work.
Independent-jaw (non-self-centering) chucks with three jaws also can be obtained.
There are hybrid self-centering chucks that have adjustment screws that can be used to further improve the concentricity after the workpiece has been gripped by the scroll jaws. This feature is meant to combine the speed and ease of the scroll plate's self-centering with the runout-eliminating controllability of an independent-jaw chuck.
Three-jaw chucks can often be found on lathes and indexing heads.
A drill chuck is a specialised self-centering, three-jaw chuck, usually with capacity of less than 0.5 in (13 mm) and rarely greater than 1 in (25 mm), used to hold drill bits or other rotary tools. This is the type of chuck that a machining layperson is most likely to be familiar with.
Some high precision chucks use ball thrust bearings to reduce friction in the closing mechanism and maximizing drilling torque. One brand name for this type of chuck, which is often used generically, is Super Chuck.
A pin chuck is a specialized chuck designed to hold small drills (less than 1 mm (0.039 in) in diameter) that could not be held securely in a normal drill chuck. The drill is inserted into the pin chuck and tightened, the pin chuck is then inserted into the larger drill chuck so that the operation can continue. Pin chucks are also found on high speed rotary tools, such as die grinders and jig grinders.
On an independent-jaw chuck, each jaw can be moved independently. Because they most often have four jaws, the term four-jaw chuck without other qualification is understood by machinists to mean a chuck with four independent jaws. The independence of the jaws makes these chucks ideal for (a) gripping non-circular cross sections and (b) gripping circular cross sections with extreme precision (when the last few hundredths of a millimeter [or thousandths of an inch] of runout must be manually eliminated). The non-self-centering action of the independent jaws makes centering highly controllable (for an experienced user), but at the expense of speed and ease. Four-jaw chucks are almost never used for tool holding. Four-jaw chucks can be found on lathes and indexing heads.
Self-centering chucks with four jaws also can be obtained. Although these are often said to suffer from two disadvantages: inability to hold hex stock, and poor gripping on stock which is oval, only the latter is true. Even with three jaw self centering chucks, work which is not of uniform section along the work (and which is not free of spiral or 'wind')should not be gripped, as the jaws can be strained and the accuracy permanently impaired.
Four-jaw chucks can easily hold a workpiece eccentrically if eccentric features need to be machined.
For special purposes, and also the holding of fragile materials, chucks are available with six or eight jaws. These are usually of the self-centering design, and may be built to very high standards of accuracy.
Two-jaw chucks are available and can be used with soft jaws (typically an aluminium alloy) that can be machined to conform to a particular workpiece.
Many chucks have removable jaws (often the top part is removable leaving the base or 'master jaw' assembled with the scroll), which allows the user to replace them with new jaws, specialized jaws, or soft jaws.
A collet, one type of chuck, is a sleeve with a (normally) cylindrical inner surface and a conical outer surface. The collet has kerf cuts along its length to allow it to expand and contract. Depending on the collet design, it can be either pulled (via a threaded section at the rear of the collet) or pushed (via a threaded cap with a second taper) into a matching conical socket to achieve the clamping action. As the collet is forced into the tapered socket, the collet will contract, gripping the contents of the inner cylinder.
Collets are most commonly found on milling machines, lathes, wood routers, and precision grinders. There are many different systems, common examples being the ER, 5C, and R8 systems. Collets can also be obtained to fit Morse or Brown and Sharpe taper sockets.
Typically collets offer higher levels of precision and accuracy than self-centering chucks, and have a shorter setting up time than independent-jaw chucks. The penalty is that most collets can only accommodate a single size of workpiece. An exception is the ER collet which typically has a working range of 1 mm (about 0.04 in).
Collets usually are made to hold cylindrical work, but are available to hold square, hexagonal or octagonal workpieces. While most collets are hardened, "emergency" collets are available that can be machined to special sizes or shapes by the user. These collets can be obtained in steel, brass, or nylon. Step collets are available that are machinable to allow holding of short workpieces that are larger than the capacity of normal collets.
Developed by Bosch in 1975 for hammer drills, the SDS uses a cylindrical shank on the tool, with indentations to be held by the chuck. A tool is inserted into the chuck by pressing in, and is locked in place until a separate lock release is used. The rotary force is supplied through wedges that fit into two or three open grooves. The hammer action actually moves the bit up and down within the chuck since the bit is free to move a short distance. Two sprung balls fit into closed grooves, allowing movement whilst retaining the bit. SDS relies on a tool having the same shank diameter as the chuck; there are three standard sizes:
Many SDS drills have a "rotation off" setting, which allows the drill to be used for chiselling. The name SDS comes from the German steck, dreh, sitzt (insert, twist, fits). German-speaking countries may use Spannen durch System (Clamping System), though Bosch uses Special Direct System for international purposes.
Used for holding ferromagnetic workpieces, a magnetic chuck consists of an accurately centered permanent magnet face. Electromagnets or permanent magnets are brought into contact with fixed ferrous plates, or pole pieces, contained within a housing. These pole pieces are usually flush with the housing surface. The part (workpiece) to be held forms the closing of the magnetic loop or path, onto those fixed plates, providing a secure anchor for the workpiece.
Commonly used for holding silicon wafers during lithography processes, an electrostatic chuck comprises a metal base-plate and a thin dielectric layer; the metal base-plate is maintained at a high-voltage relative to the wafer, and so an electrostatic force clamps the wafer to it. Electrostatic chucks may have pins, or mesas, the height of which is included in the reported dielectric thickness; a design by Sandia National Laboratory uses a patterned silicon-dioxide dielectric to form the pins.
A vacuum chuck is primarily used on non-ferrous materials, such as copper, bronze, aluminum, titanium, plastics, and stone. In a vacuum chuck, air is pumped from a cavity behind the workpiece, and atmospheric pressure provides the holding force. Vacuum produces a hold down force of 14.7 psi (101 kPa) at sea level, decreasing at higher elevations where the atmospheric pressure is lower. The use of vacuum chucks is increasing.